1. Field of the Invention
The present invention relates generally to a stirring device. More particularly, the invention relates to a stirring device suitable for use in processing, such as mixing, dissolving, dispersing or so on of medium or high viscous fluid or the like, in a chemical industry, a pharmaceutical industry, a food industry and so on.
2. Description of the Related Art
Conventionally, a stirring device including a stirring vessel, a rotary shaft rotatably supported within the stirring vessel and stirring vanes fixed on the rotary shaft for rotation within the stirring vessel, has been widely used in fields of a chemical industry, a pharmaceutical industry, a food industry or the like. Amongst, as typical stirring devices to be used for process, such as mixing, dissolving, dispersing or so on of medium or high viscous fluid or the like, stirring devices having helical ribbon vanes, anchor vanes or the like have been used.
The helical ribbon vane is a stirring vane which forces a liquid to be stirred to circulate vertically and in conjunction therewith to prevent the liquid to be stirred from stagnating near an inner peripheral wall of a container, by tilting the vanes.
On the other hand, the anchor vane having a vane diameter substantially equal to a diameter of the vessel is advantageous in the case where a peripheral interface layer is disturbed or solid is deposited on the peripheral surface. Therefore, it is suitable for stirring a high viscous fluid or crystallizing operation.
Moreover, as recent technologies, (1) Japanese Patent Application Laid-open No. 61-200842 (1986) discloses a stirring device which has a stirring shaft carrying a bottom paddle arranged in a bottom portion of a vessel in sliding contact with a bottom wall surface of the stirring vessel and a grid vane located above the bottom paddle and constituted of an arm portion and a strip extending perpendicularly from the arm portion, and a plurality of baffle plates provided on a side wall surface of the stirring vessel and aligned in an axial direction from the lower portion to the upper portion in a spaced apart relationship.
Also, (2) Japanese Patent Application Laid-open No. 5-49890 (1993) discloses a stirring device which has a rotary shaft extending vertically at the center portion within a vertical-cylindrical stirring vessel, mounting a plurality of rectangular paddle vanes in multiple stages in a vertical direction, and placing the lowermost paddle vane advanced in a rotating direction with respect to the lower stage paddle vane vertically adjacent with the lowermost paddle vane in a cross axis angle less than 90.degree..
Furthermore, (3) Japanese Patent Application Laid-open No. 7-124456 (1995) discloses a stirring device which comprises a vertical cylindrical stirring vessel, a stirring shaft extending vertically to a center in the stirring vessel for rotation from outside of the stirring vessel, a plurality of upper vanes mounted on the stirring shaft, formed in the form of trapezoid-shape and having a recess in the lower center portion, lower vanes mounted perpendicularly to the upper vanes, formed into rectangular shape and having a recess in the upper center portion, and backwardly slanting vanes coupled with the lower vanes integrally or separately, slanted in a reverse direction relative to a rotating direction and formed with cut-out portions symmetrically positioned about the stirring shaft.
However, among such conventional stirring devices, the stirring device having the helical ribbon vane and the stirring device having the anchor vane may encounter a problem that a doughnut-ring-like not well mixed portion can be generated in stirring at a particular rotation speed depending on a nature of fluid as contents.
It should be noted that the stirring devices (1) to (3) above encounter the following problems. The stirring device of (1) employing the bottom vane and grid vane has a low withdrawal effect at the upper center portion. The stirring device of (2) employing a plurality of rectangular paddle vanes and the stirring device of (3) employing generally trapezoidal vane tend to partially interfere an upward flow from the lower side of the stirring vanes by a discharge flow from the upper side of the stirring vanes to hinder optimal stirring.
In the recent year, according to progress of increasing of kinds of products to be manufactured and reducing of the number of products to be manufactured in one lot, a demand for a higher performance and wider variation in the stirring device has been growing. Namely, there has been required a stirring device which can achieve a high stirring efficiency, can be easily washed and maintained and has smaller mechanical load. However, none of the conventional stirring devices can satisfy such requirements.